Infants learn language not only through sound, but also through lip reading, according to a new study published in the journal Proceedings of the National Academy of Sciences.

Researchers say the new findings defy the conventional view that babies learn to speak through sound alone and the research may even assist in diagnosing autism spectrum disorders in the future.

Scientists from Florida Atlantic University studied 89 infants ranging in age from 4 months to 12 months old. They also studied 21 adults. Participants watched a 50-second video of a woman reciting a monologue in their native English, while researchers used an eye tracker to determine where they directed their pupils while watching and listening to the video.

Four-month-old infants, along with adults, spent more time looking at the speaker's eyes. But babies between 6 and 12 months shifted their attention between the speaker's mouth and eyes. It is unknown at what age the shift from mouth to eyes is totally complete, but the older infants tended to look at the speaker's eyes more than her mouth.

"By this time at 12 months, babies are already producing their first words and have mastered the first sounds and structures of the language," said David Lewkowicz, an expert on infant perceptual development and lead author of the study. "They no longer have to lip-read as they ramp up their first speech patterns and they are free to shift back to the eyes, where you find a great deal of social information.

"The eyes are the window to the brain, and by looking at the eyes, we are able to know what the other person is thinking and what they want, their desires," he said.

"This research confirms and extends what we already know about typical development: Language is not just an auditory process, it is the integration of visual and auditory information as the child learns words," said Stephen Camarata, professor of hearing and speech sciences at Vanderbilt University. "What happens in infancy has important ramifications for later development."

Researchers said the data could contribute to autism research, as well. Two-year-old children with autism attend more to speaker's mouths, according to past literature on the developmental disorder. This study shows that attention to the mouth is a normal developmental phase during the first year, and the comparision could aid in autism diagnosis at an earlier age.

"Right now, the earliest one can diagnose a child with autism is 18 months, so this could possibly be a way in the future to diagnose infants as early as 12, 13 or 14 months if we find babies are not making a shift back to the eyes around this age," Lewkowicz said. "If that is the case, this would be a huge step forward in the development of diagnostic tools for autism because it would be six months earlier than what we can do now.

"Because the brain is so elastic and there is an enormous proliferation of neuro structure during infancy, if we could pick up these difficulties as early as 12 months, we could begin to intervene at an earlier time and get far better outcomes in children," he said.

But Rhea Paul, director of the Yale Child Study Center's Laboratory of Development Communication Disorders, warned that an autism diagnosis is not so easily simplified, and it could be dangerous to make blanket claims regarding autism and eye attention.

"I think caution is always warranted when attempting to identify a single early 'marker' for a developmental disorder, such as failure to go back to looking at eyes at 12 months as a sign of autism, simply because of the great variability in behavior and development that is normal in the first couple of years of life," Paul told ABCNews.com.

Thirty percent of children that researchers attempted to test had to be eliminated for a variety of reasons.